iwlwifi: push virtual interface through

Rather than keeping every bit of information
around in priv and the virtual interface, add
a virtual interface to many functions and use
the information directly from it.

This removes beacon_int, assoc_capability and
assoc_id from struct iwl_priv.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>